Re: [Xen-devel] [PATCH 7/7] xen: sched_rt: print useful affinity info when dumping

[Meng Xu <xumengpanda@xxxxxxxxx>]

>>>
>>> This change also takes the chance to add a scratch
>>> cpumask, to avoid having to create one more
>>> cpumask_var_t on the stack of the dumping routine.
>>
>> Actually, I have a question about the strength of this design. When we
>> have a machine with many cpus, we will end up with allocating a
>> cpumask for each cpu. Is this better than having a cpumask_var_t on
>> the stack of the dumping routine, since the dumping routine is not in
>> the hot path?
>
> The reason for taking this off the stack is that the hypervisor stack is
> a fairly limited resource -- IIRC it's only 8k (for each cpu).  If the
> call stack gets too deep, the hypervisor will triple-fault.  Keeping
> really large variables like cpumasks off the stack is key to making sure
> we don't get close to that.

I see. I didn't realize the fact of the limited size of hypervisor
stack. That makes sense.


Thank you very much for clarification! :-)

Best,

Meng

-----------
Meng Xu
PhD Student in Computer and Information Science
University of Pennsylvania

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